The present invention relates generally to control systems for internal combustion engines and, in particular, to a method for operating homogeneous charge compression ignition (HCCI) engines at low speed and low load.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke. Both traditional gasoline engine and diesel engine combustion involve premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies or concepts have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI) engine, although in practice the intake charge is not necessarily homogeneous. HCCI is a distributed, flameless, autoignition combustion process that is controlled by the oxidation chemistry, rather than by fluid mechanics. In a HCCI engine, the intake charge is usually nearly homogeneous in composition, temperature, and residual level at the time of intake valve closing. The intake charge consists of a fuel/air mixture that is introduced into the combustion cylinder, as in spark-ignition engines, but the fuel/air mixture is heated prior to and/or during its introduction into the cylinder and, therefore, auto-ignites during the compression stroke, as in diesel engines but normally without the injection of fuel into the cylinder near TDC of the compression stroke. Because HCCI is a distributed kinetically controlled combustion process HCCI engines can, and indeed must, operate with a very dilute fuel/air mixture having a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for HCCI is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are eliminated. Because of this very dilute fuel/air mixture, a HCCI engine can operate unthrottled with diesel-like fuel economy.
At medium engine speed and load, a combination of valve timing strategy and exhaust rebreathing (the use of exhaust gas to heat the intake charge entering a combustion space in order to encourage autoignition) during the intake stroke has been found to be very effective in providing adequate heating to the intake charge so that autoignition during the compression stroke leads to stable combustion with low noise. This method, however, does not work satisfactorily at or near idle speed and load conditions. As the idle speed and load is approached from a medium speed and load condition, the exhaust temperature decreases. At near idle speed and load there is insufficient energy in the rebreathed exhaust to produce reliable autoignition. As a result, at the idle condition, the cycle-to-cycle variability of the combustion process is too high to allow stable combustion. Consequently, one of the main difficulties in operating HCCI engine has been to control the combustion process properly so that robust and stable combustion with low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions. The benefits of HCCI combustion have been known for many years. The primary barrier to product implementation, however, has been the in ability to control this process.
It is desirable, therefore, to provide a means for operating a HCCI engine efficiently and with stable combustion at low speeds and loads. It is also desirable to control the combustion process properly in a HCCI engine so that robust and stable combustion with low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions.
The present invention concerns a method for operating a multi-cylinder homogeneous charge compression ignition (HCCI) engine at low engine speeds and loads. The method is performed on an operating HCCI engine having multiple activated cylinders. The method begins at the step of monitoring the engine operating speed and the individual cylinder loads of the HCCI engine having multiple activated cylinders in a stable combustion condition. The cylinder load is directly related to the rate of fuel addition to the cylinder, a quantity which is commanded by the engine performance control system to achieve desired vehicle performance, and which is the same for all active cylinders. The rate of fuel addition to the cylinder, therefore, is one measure of the cylinder load. Those skilled in the art, however, can appreciate that many means for measuring the cylinder load can be utilized while remaining within the scope of the present invention.
The decision on whether to activate or deactivate cylinders is based on an area of th e speed-load map of the engine where it has been previously determined that stable HCCI operation of the engine is possible. Both the engine speed and load of the engine, therefore, must be taken into account when deciding whether to activate or deactivate cylinders. Since the total engine load is equal to the sum of the loads on each of the individual cylinders, and the load on each active cylinder should be the same, it may be necessary to measure the load on only one active cylinder, or to compute an average over the active cylinders, in order to determine at which point the engine is currently operating on its speed-load map.
When the cylinder load drops below a predetermined value equal to the lowest allowed load at its current engine speed, at least one cylinder is deactivated and the fueling rate to the remaining active cylinders is increased to maintain the stable combustion in the remaining activated cylinders while satisfying the requirements of the engine control system. This predetermined value equal to lowest allowed load may be different for different engine speeds. When the active cylinder load exceeds another predetermined value at its current engine operating speed, the at least one cylinder is reactivated. This another predetermined value may be different for different engine speeds.
Preferably, the method according to the present invention utilizes an engine control system to monitor the speed and load of the cylinders, to activate the cylinders and to deactivate the cylinders. One means for deactivating the unneeded cylinders is to stop the flow of fuel and to keep both the intake and exhaust valves closed on the cylinders to be deactivated. This eliminates pumping losses due to intake into and exhaust from the deactivated cylinders. Another means for deactivating the unneeded cylinders is to stop only the flow of fuel to the cylinder, which would advantageously deactivate the cylinder but would disadvantageously include pumping losses. Those skilled in the art, however, can appreciate that many means for deactivating combustion cylinders can be utilized while remaining within the scope of the present invention.
The method according to the present invention involves the use of cylinder deactivation at and near idle in a compression ignition premixed combustion engine, so that only some of the cylinders in the HCCI engine are firing, thereby increasing the effective load of the remaining firing cylinders, which advantageously allows the engine to remain operating in a stable combustion condition. The remaining activated cylinders are operated at a higher load condition at and near idle than would be the case if all cylinders were operating.
In operation, the multi-cylinder HCCI engine is operated normally, but the speed of the engine and the loads of the cylinders are monitored by the engine control system. If the cylinder load drops below the lowest allowed load at its current engine speed, the engine control system will deactivate a predetermined cylinder or cylinders. If the cylinder load exceeds the lowest allowed load at its current engine speed after the cylinder or cylinders has been deactivated, the engine control system maintains the current deactivated operation. If the cylinder load again drops below the lowest allowed load at its current engine speed, the engine control system will deactivate another predetermined cylinder or cylinders. The engine control system constantly monitors the engine speed and the cylinder load and activates and deactivates cylinders as necessary. The control system will continue to deactivate cylinders up to a predetermined number of cylinders that leaves activated the minimum number of cylinders that allow the engine to be run in a stable combustion condition. If the engine is operating with the minimum number of cylinders and the speed and the cylinder load remains below its lowest allowable load, the control system switches the engine to an alternate operating mode, such as (but not limited to) typical spark-ignited engine operation. The control system then monitors the engine speed and cylinder load until a range of speed and load is reached where HCCI operation can resume.
By incorporating HCCI operation into an engine, the engine can be developed with xe2x80x9clower-boundxe2x80x9d NOx and particulate emissions, resulting in an engine that is nearly as efficient as a diesel with greatly reduced emissions constraints or a gasoline engine with fuel economy approaching that of a diesel engine. By incorporating the method according to the present invention into the control system of an HCCI engine, the HCCI engine can be advantageously operated with the combustion process controlled properly so that robust and stable combustion with low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions, including low engine speed, low total engine load and low cylinder load.